The semiconductor device manufacturing industry has applied a well known technique used in embroidery and other needle work to support semiconductor wafers during typical manufacturing processes, such as mechanical sawing operations. To support a semiconductor wafer during such a sawing operation, the wafer is mounted to an adhesively coated surface of a tightly stretched polymer sheet. One of the techniques adapted by the semiconductor industry for stretching the polymer sheet to a tight and substantially rigid support surface is a double hoop frame structure which is well known as a useful tool in the embroidery art, for example. The sheet of material is stretched and tensioned over the opening of an inner hoop. An outer hoop is fitted concentrically over the material into a coplanar position with the inner hoop. The size of the outer hoop is such that the material becomes clamped between inner and outer adjacent surfaces of the two hoops when they are located in their coplanar position.
In adapting the well known hoop principle to providing a support for an article such as a semiconductor wafer, the tightness of the polymer sheet and the physical dimensions of the hoops as frame members become critical. Particularly when the mounting frame formed by two concentric frame members and the adhesively coated polymer sheet support a semiconductor wafer during a sawing operation, the continued rigidity of the adhesively coated sheet throughout the operation is important. Also, the polymer sheet, unlike cloth material is typically smooth on one surface and exhibits a low coefficient of friction between that surface and an adjacent frame member. Low friction characteristics of the polymer sheet and a relatively thin material thickness of the sheet have caused prior art mounting frames to feature annular holding protrusions which are alternately positioned along the oppositely adjacent surfaces of the two frame members, so that the two frame members assume a defined, seated position with respect to each other.
The tensioning of the polymer sheet or film is simple because of the adhesive coating on only one surface of the film. In mounting the film to the frame, a properly sized piece of the film material is placed over the inner frame member with the adhesively coated surface directed upward or away from the inner frame member. The outer frame member is then placed concentrically over the inner one, such that a base surface of the outer frame member contacts the adhesively coated surface of the film. As the outer frame member is pushed down over and into coplanar position with the inner frame member, the adhesively coated surface of the film attaches itself to the outer frame member so that the film becomes tightened as the outer frame member is pushed into the coplanar position with respect to the inner frame member. The surface of the film opposite the adhesively coated surface slides easily relative to the inner frame member during the positioning of the two frame members with respect to each other, so that the film is uniformly tensioned when the two frame members are located in coplanar relationship with respect to each other.
The semiconductor wafer is then attached to the outwardly directed adhesive surface of the tightly stretched film in preparation for a sawing operation. During the sawing operation a cutting wheel of a sintered material containing diamond particles moves along straight lines of a grid pattern on the surface of the wafer to cut the wafer into individual, rectangular shaped semiconductor devices. The individual devices remain attached to the film after the sawing operation in the original array in which they were formed in the wafer. In such array they are tested, and the good devices are selectively removed from the array by a typical selective pick and place operation.
Problems in the referred to sawing operation result in occasional breakage of saw blades. The cost of replacing saw blades then adds significantly to the cost of the semiconductor devices. The blades are of a specially hardened material and are machined to a high degree of precision. A typical, state of the art saw blade has a thickness of 0.002 inch which is balanced to spin at typically 32,000 RPM. A likely cause for a saw blade to break has been identified as a contact by the blade with the outer frame member of the referred-to mounting frame.
The height of the saw blade is precisely adjusted to cut entirely through the thickness of the wafer which is typically 0.020 inch thick. The film which is typically 0.005 inch thick may become slightly scored but is not cut during the sawing operation. Thus, an observed occasional contact by the saw blade with the outer member of the mounting frame is attributed to a creeping movement by such outer member of the frame with respect its inner member during the sawing operation. It is suspected that tension forces in the film combine with high speed vibrational energy to cause an upward movement of the outer frame member into the path of the moving saw.